What's the best way to implement a single tri-state checkbox within angularjs?

Question

Is there a simple way to place a single tri-state checkbox on a web-page and bind it to a boolean model so the latter can take true, false or null values?

The closest solution I found so far is http://jsfiddle.net/HB7LU/454/ but it has a flaw when setting up an initial view state (as there is no way to get a model value during first rendering). Any other suggestions deal with multiple child checkboxes and solves the problem by watching on them.

Answer 1

http://jsfiddle.net/xJhEG/ I made it in a commercial project. Tristates are true, false, null (not "unknown")

.directive('indeterminate', [function() {
    return {
      require: '?ngModel',
      link: function(scope, el, attrs, ctrl) {
        var truthy = true;
        var falsy = false;
        var nully = null;
        ctrl.$formatters = [];
        ctrl.$parsers = [];
        ctrl.$render = function() {
          var d = ctrl.$viewValue;
          el.data('checked', d);
          switch(d){
          case truthy:
            el.prop('indeterminate', false);
            el.prop('checked', true);
            break;
          case falsy:
            el.prop('indeterminate', false);
            el.prop('checked', false);
            break;
          default:
            el.prop('indeterminate', true);
          }
        };
        el.bind('click', function() {
          var d;
          switch(el.data('checked')){
          case falsy:
            d = truthy;
            break;
          case truthy:
            d = nully;
            break;
          default:
            d = falsy;
          }
          ctrl.$setViewValue(d);
          scope.$apply(ctrl.$render);
        });
      }
    };
  }])

Answer 2

Here my Fiddle, starting from TruongSinh and changing

http://jsfiddle.net/xJhEG/25/

without

var truthy = true;
var falsy = false;
var nully = null;

Answer 3

You have take advantage of the indeterminate state of <input type="checkbox">.

MDN web docs: There exists an indeterminate state of checkboxes, one in which it is not checked or unchecked, but undetermined. This is set using the HTMLInputElement object's indeterminate property via JavaScript (it cannot be set using an HTML attribute).

PLUNKER: TRISTATE DIRECTIVE

HTML

<label>
    <input type="checkbox" ng-model="state" indeterminate /> {{state}}
</label>

DIRECTIVE

app.directive('indeterminate', function() {
  return {
    restrict: 'A',
    scope: {
      model: '=ngModel'
    },
    link: function(scope, el, attrs, ctrl) {
      var states = [true, false, undefined];
      var index = states.indexOf(scope.model);
      setIndeterminate();

      el.bind('click', function() {
        scope.model = states[++index % 3];
        setIndeterminate();
      });

      function setIndeterminate() {
        scope.$applyAsync(function() {
          el[0].indeterminate = (scope.model === undefined);
        });
      }
    }
  };
});

Answer 4

I've created directive, which you can use. Three-state checkbox AngularJS Directive on GitHub

There is also a post, how it was built: Creating Angular Directive "Three-state checkbox

You can try a DEMO

And the directive looks like that:

angular.module("threeStateCheckbox", [])
    .directive("threeStateCheckbox", ['$compile', function($compile){
        return {
            restrict: "A",
            transclude: true,
            require: 'ngModel',
            link: function(scope, element, attrs, ngModel){
                var states = [true, false, null];
                var classNames = ["checked", "unchecked", "clear"];
                scope.click = function(){
                    var st;
                    states.map(function(val, i){
                        if(ngModel.$modelValue === val){
                            st = states[(i+1)%3];
                        }
                    });
                    ngModel.$setViewValue(st);
                    ngModel.$render();
                };
                scope.tscClassName = function(){
                    var className;
                    states.map(function(val, i){
                        if(ngModel.$modelValue=== val){
                            className = classNames[i];
                        }
                    });
                    return className;
                };
                element.attr("class", "tri-sta-che ");
                element.attr("ng-click", "click()");
                element.attr("ng-class", "tscClassName()");
                element.removeAttr("three-state-checkbox");
                $compile(element)(scope);
            }
        };
    }]);

Answer 5

Because all previous answers don't work since AngularJS 1.7 (the checkbox model only allows boolean values now and everything else gets converted to boolean), I now found a solution that works until v1.8: Thanks to @The.Bear for the base.

To use it, simply include <tristate label="someOptionalText" ng-model="yourVariableToBindTo" /> and the corresponding directive.

var app = angular.module('tristatedemo', []);

app.controller('MyCtrl', function() {
  this.state = null; //initial state
});

app.directive('tristate', function() {
  return {
    restrict: 'E',
    scope: {
      model: '=ngModel',
      label: '@'
    },
    template: '<input type="checkbox" /> {{label}}',
    link: function(scope, el, attrs, ctrl) {
      var states = [true, false, null];
      var index = states.indexOf(scope.model);
      setIndeterminate();

      el.bind('click', function() {
        scope.model = states[++index % 3];
        setIndeterminate();
      });

      function setIndeterminate() {
        scope.$applyAsync(function() {
          var cb = el.find('input')[0];
          cb.checked = scope.model;
          cb.indeterminate = (scope.model === null);
        });
      }
    },
  };
});

<!DOCTYPE html>
<html ng-app="tristatedemo">

<head>
  <script data-require="angular.js@1.8.x" src="https://cdnjs.cloudflare.com/ajax/libs/angular.js/1.8.2/angular.min.js" data-semver="1.8.2"></script>
</head>

<body ng-controller="MyCtrl as ctrl">
  {{(ctrl.state === null) ? "null" : ctrl.state}}<br>  <!-- only for demo -->
  <tristate label="abc" ng-model="ctrl.state" />
</body>

</html>